New Information on Segisaurus Halli, a Small Theropod Dinosaur from the Early Jurassic of Arizona

Here we redescribe the holotype and only specimen of Segisaurus halli, a small Early Jurassic dinosaur and the only theropod known from the Navajo Sandstone. Our study highlights several important and newly recognized features that clarify the relationships of this taxon. Segisaurus is clearly a primitive theropod, although it does possess a tetanuran-like elongate scapular blade. Nonetheless, it appears to be a coelophysoid, based on the presence of a pubic fenestra, a long and ventrally curved pubis, and some pelvic (and possibly tarsal) fusion. Segisaurus does possess a furcula, as has now been observed in other coelophysoids, thus strengthening the early appearance of this ‘avian’ feature. The absence of an external fundamental system in bone histology sections and the presence of sutural contact lines in the caudal vertebrae, scapulocoracoid, and (possibly) between the pubis and ischium support the inference that this specimen is a subadult, neither a true juvenile nor at full skeletal maturity. A cladistic analysis confirms Segisaurus as a coelophysoid theropod. Poor resolution within Coelophysoidea makes speculation about evolution in this clade difficult, but Segisaurus apparently represents a lineage that had been distinct from Coelophysis and Syntarsus since at least the Carnian.